A quantum cryptographic communication implies a communication system for sharing secret information on a transmitter side and a receiver side, in which the transmitter side modulates quanta, generally speaking, photons based upon the secret information and then transfers the modulated photons, and then, the receiver side demodulates the received quanta. Security of the secret information modulated on the quanta is guaranteed based upon the uncertainty principle of quantum mechanism. It should be noted that since quanta can be easily destroyed and all of information cannot be firmly transferred, random number information is employed as the secret information. The random number information which is transferred between the transmitter side and the receiver side is employed as a cryptographic communication-purpose secret key after a data processing operation such as correcting errors and improving secrecy has been carried out with respect to the random number information. To be more specific, this technical idea is referred to as a quantum key distribution.
On the other hand, a phase modulating system corresponds to such a modulating system which is suitably employed in quantum cryptography in which optical fiber communication is employed as a base. This reason is given as follows. That is, relative phase information of time difference twin photon pulses called as a signal optical pulse and a reference optical pulse may be comparatively held while being transferred through an optical fiber. The relative phase information may be demodulated by monitoring an interference phenomenon which occurs when the signal optical pulse is multiplexed with the reference optical pulse. As a result, generally speaking, a phase modulation type quantum cryptographic communication apparatus has such a structure that an optical path loop which is called as an asymmetrical Mach-Zehnder interferometer used to generate twin photon pulses is provided on the side of a transmitter apparatus, whereas another asymmetrical Mach-Zehnder interferometer having the same dimension as that of the first-mentioned Mach-Zehnder interferometer is provided on the side of a receiver apparatus, which multiplexes and causes the twin photon pulses to interfere with each other.
However, in the quantum cryptographic communication apparatus having the above-explained structure, the below-mentioned two fluctuation effects may give adverse influences which cannot be neglected. That is, one fluctuation effect corresponds to a birefringent polarized wave fluctuation which is in a communication path, and another fluctuation effect corresponds to an optical path length fluctuation which occurs between the two asymmetrical Mach-Zehnder interferometers provided with the transmitter apparatus and the receiver apparatus. Since these two fluctuations occur, in the quantum cryptographic communication apparatus having the above-explained structure, both the polarized waves and the optical path lengths must be continuously adjusted.
With respect to the above-explained fluctuation problem, defect solutions have been progressed. In a quantum cryptographic communication called as a plug & play system, an automatic compensation of polarized wave fluctuations is realized by canceling polarized wave fluctuations effected on a quantum transfer path in a going path and a returning path, since twin photon pulses are reciprocated from a quantum receiver apparatus to a quantum transmitter apparatus and from the quantum transmitter apparatus to the quantum receiver apparatus, and polarization planes of the respective optical pulses are rotated at a right angle in a non-reciprocal manner within the quantum transmitter apparatus, and then, the rotated optical pulses are reflected. Also, an optical path loop which is used so as to generate twin photon pulses having a time difference between signal light and reference light within the quantum receiver apparatus is made identical to an optical path loop which is used so as to multiplex and cause the twin photon pulses to interfere with each other in the quantum receiver apparatus, so that the stable multiplexing and interfering effects capable of compensating the optical path length fluctuations may be realized (refer to, for example, Patent Documents 1 and 3, and Non-patent Document 1).
In the above-described plug & play type quantum cryptographic communication apparatus, in particular, as one structural example of an optical system which constitutes the quantum transmitter apparatus, although not specified to a quantum cryptography-purpose optical system, such an optical system using a polarization rotating mirror may be arranged (refer to, for instance, Patent Document 2).
Patent Document 1: JP 2002-289298 A (paragraph 0033, FIG. 1)
Patent Document 2: JP 5-241104 A (paragraph 0012, FIGS. 4(a) and 5(b))
Patent Document 3: U.S. Pat. No. 6,188,768 B1 (FIGS. 2 and 4, and page 6)
Non-patent Document 1: G. Ribordy, et. al. “Automated “Plug & Play” Quantum Key Distribution” Electronics Letters 34, (22), pp. 2116 to 2117, 1998
The conventional plug & play type quantum cryptographic communication apparatus constitutes a stable system with respect to the fluctuations by reciprocating the photon pulses on the same optical path. As a result, the photon pulses must pass twice through the phase modulator employed in the quantum receiver apparatus in the going path and the returning path. While the repetition frequency of the light source is increased in order to realize the high communication speed, if the photon pulses are oscillated within a shorter time period than a time duration, during which the photon pulses are reciprocated within the optical path, then the timing at which the photon pulses pass through the phase modulator in the going path is approximated to the timing at which the photon pulses pass through the phase modulator in the returning path within a single time period. As a result, there is such a problem in that even when only the photon pulses in the returning path are tried to be phase-modulated, the photon pulses in the going path are also phase-modulated which do not require such the phase modulation, depending upon a selected repetition frequency.
The present invention has been made to solve the above-explained problem, and therefore, has an object to obtain a quantum cryptographic communication apparatus capable of avoiding that a photon pulse in a going path is phase-modulated, and also capable of freely selecting a repetition frequency of a light source in order to increase a communication speed.